Spread spectrum communications are commonly used in cellular networks that provide wireless service to mobile stations, such as wireless telephones. Such cellular networks typically operate in accordance with IS-95 CDMA or cdma2000 standards.
The signals transmitted by base stations in a spread spectrum communication system are spread by a pseudonoise (PN) code. For example, in accordance with IS-95 CDMA standards, the pilot signal transmitted by a base station is spread by a “short” PN code that has a register length of 15 and a chipping rate of 1.2288 MHz. Each pilot signal may be spread by the same “short” PN code but with a different phase or “PN offset.” Thus, different pilot signals may be distinguished by their PN offsets. In accordance with IS-95 CDMA standards, each PN offset is defined by an index that is an integer in the range of 0 through 511. The actual PN offset is then found by multiplying its index by 64 chips.
A mobile station may find base stations, or sectors of base stations, by detecting their pilot signals. The mobile station may then keep track of the pilot signals that it has detected by maintaining “pilot sets” that identify sectors by the PN offsets of their pilot signals. For example, a mobile station may maintain an “active set” that identifies the PN offsets of the sectors with which the mobile station is currently communicating, a “neighbor set” that identifies the PN offsets of neighboring sectors, a “candidate set” that identifies the PN offsets of sectors that are candidates for inclusion in the active set, and a “remaining set” that identifies all other PN offsets that the mobile station has detected.
The mobile station may continually monitor the signal strengths of the pilot signals it has detected in order to adjust the placement of the pilot signals in these sets. For example, when the signal strength of a pilot signal in the “neighbor set” or “remaining set” exceeds a threshold value, the mobile station may add the pilot signal to its “candidate set” and report its signal strength to the network by transmitting a Pilot Strength Measurement Message (PSMM). The network may subsequently instruct the mobile station to effect a handoff by moving the sector from the candidate set to the active set. Thus, the neighbor set and remaining set identify sectors that may be available for a handoff at some later time.
A mobile station may populate its neighbor set based on neighbor list messages that are broadcast by the mobile station's serving sector. However, a mobile station typically populates its remaining set by scanning for pilot signals. Moreover, a mobile station typically scans for pilot signals by using a PN increment that defines which of the 512 possible PN offset indices are actually used in the cellular network. For example, a PN increment of N would indicate that only the PN offset indices that are multiples of N are used. Thus, when scanning for pilot signals, the mobile station would only need to look for PN offsets with indices N, 2N, 3N, etc.